This invention is in the field of optical measurements and imaging. This invention relates to methods for making extremely accurate measurements in a medium by continuously measuring the index of refraction of the medium, such as water or biological tissue.
The index of refraction of a medium is the ratio of the speed of light in a vacuum to the speed of light in the medium. This is an important parameter, as the index of refraction not only changes the time it takes for the light to travel through the medium, but also can change the angle of propagation. This is highly important when making very precise optical measurements in a medium that has a variable index of refraction. For instance, when taking a 3D laser scan of a scene at the bottom of the ocean, the correct index at that depth and temperature are needed in order to measure range and angle accurately and thus create a dimensionally correct 3D image.
Austin and Halikas [R. W. Austin and G. Halikas, “The index of refraction of seawater”, SIO Ref 76-1 (Scripps Institution of Oceanography, La Jolla, Calif., 1976)] presented extensive tables and interpolation algorithms for the index of refraction of seawater as a function of wavelength of light, salinity, temperature, and pressure. The index of refraction of seawater varied from 1.329128 to 1.366885. In addition, suspended particulate matter in sea water changes the index of refraction. Living phytoplankton typically have indices of refraction in the range of 1.01 to 1.09 relative to the index of refraction of seawater. Detritus and inorganic particles generally have indices in the range of 1.15 to 1.2 relative to seawater.
When using light to measure distance in a medium, the time it takes for the signal to reach the target and return is measured and then used to calculate distance. The simple equation isR=cT/2n  (1)where R=range to target, c=speed of light in a vacuum, n is the index of refraction of the medium, and T is the measured time for the signal to travel to the target and back.
One method is to arbitrarily choose the index to be the average of the 2 extreme values stated above (1.348) when calculating range using equation 1. Now assume the actual index is 1.329128 and the distance to the target is 10 m. If range is calculated using the average index (1.348), the range is computed as 9.86 m, a 14 cm (˜5″) error. The methods and devices describe herein eliminate this error by constantly measuring the index of refraction and using the updated index in the primary measurement device.
In addition to range errors, a varying index will also cause angle errors. When light passes from one medium (air in the instrument housing) to the medium (ocean water) the light refracts by Snell's law:n1 sin ⊖1=n2 sin ⊖2  (2)where n1 is the index of the medium in the housing, ⊖1 is the angle of incidence (compared to normal) of the beam as it passed from n1 to n2, n2 is the index of the medium, and ⊖2 is the angle of the beam (compared to normal) as it propagates through the medium.
In a scanning system this equates to spatial offsets. In triangulation, stereoscopic vision, photometric stereo, photoclinometry, stereo-photoclinometry, and other angular based measurement systems this equates to angle and range offsets. For instance, a beam with a 15° angle from normal exiting an optical instrument and travelling 10 m in water (index=1.348) will produce a spot 1.96 m from normal at the 10 m range. In comparison, a beam with a 15° angle from normal exiting an optical instrument and travelling 10 m in different seawater (index=1.366885) will produce a spot 1.93 m from normal at the 10 m range—an error of greater than 1 inch. The methods and devices described herein eliminate these errors.
The current underwater 3D inspection technology is sonar. This technology is used for oil and water pipeline inspection, underwater construction, bridge inspection, hydroelectric dam inspection, and tunnel inspection. However surveyors currently need higher resolution 3D imaging to perform several underwater tasks and sonar cannot reach the higher resolutions.
Several papers and patents discuss underwater laser scanning, laser triangulation, and photometric stereo systems to provide high resolution 3D data. Most of these papers and patents show lower resolution data, or high resolution data in a lab environment. When making real world measurements working at various depths in various oceans, the index of refraction will vary by up to 3% causing errors as shown above. None of this prior art discusses the effects of index of refraction variation on the resolution of the system, much less how to correct for it.